This invention relates to a method of forming a multicoat on a substrate using a photocurable base coat composition and a top coat composition.
Multi-layered coatings having a colored base coat and a clear or transparent top coat superimposed on the base coat find many important applications in finishing a wide variety of objects for decorative and protective purposes. A method of forming such multicoat includes the steps of applying a photocurable colored base coat composition on a substrate, photocuring the base coat, applying a clear top coat composition on the base coat, and then thermally curing the entire multicoat. This method can prevent the base coat and the top coat from being intermixed prior to curing and thus give an excellent finish, while the length of total curing time may be greatly saved without subjecting the substrate to repeated heating. However, since the base coat is colored and relatively opaque to an activating light, it is imperative for the base coat composition to contain a relatively large amount of a photocuring accelerator such as N,N-dialkylaminobenzoic acid or other tertiary amines. The presence of such accelerator in the cured coating film is not desirable because it adversely affects the water-resistance and the alkali-resistance of the finished coat and the accelerator tends to migrate toward the outer surface resulting in impaired finish upon aging.
Japanese Laid Open Patent Applications Nos. 22536/78, 22537/78 and 22538/78 disclose an accelerator consisting of a reaction product of N,N-dialkyl-p-aminobenzoic acid with an epoxy resin, a polyalkylene glycol or a polyester resin. This accelerator has shown to be unsatisfactory when used in the photocurable base coat composition because it tends to decrease the curability of the coated film and serves as a plasticizer so that the resulting coat exhibits poor chemical and physical characteristics.
Japanese Laid Open Patent Application No. 100498/79 discloses a photo-crosslinkable binder consisting of a reaction product of an epoxy resin, ammonia or an alphatic or alicyclic amine, benzoylbenzoic acid and acrylic or methacrylic acid. When this binder is used particularly in colored opaque coating composition, it decreases the curability of the composition and discolors the finished coat when the coat is baked at a temperature above 200.degree. C.
It is, therefore, an object of the present invention to provide a method of forming a multicoat on a substrate using a photocurable base coat composition and a top coat composition which is free from the above-described problems. Other objects and advantages of the present invention will be apparent to those skilled in the art as the description proceeds.